Mine is the 6.7 with the 3.73 rear end and the 68RE six speed automatic. I am in love with how this truck tows this Montana. Like Okie Guy, though, Vicki and I do miss the Ford interior and the backup sensors (not available in a Dodge) but otherwise this truck is awesome. It just does not breathe hard towing this Montana. We've not towed with this truck up more than an 8% grade but the temperature up that grade, or any grade, never exceeded 205 degrees. Normally it runs at 195 with 200 being not unusual on a hot day uphill.
Ours is just short of 26,000 miles with 9,000 of that towing. I'd give you exact mile numbers but my log is out in the truck.
I'm not sure mine is a valid comparison to what you are looking at, though. The Dodge you are looking at should have mpg numbers superior to the other brands of that era, too.